Fabrication, Comparison, Optimization, and Applications of Conductive Graphene Patterns Induced via CO2 and Diode Lasers

Fabrication of conductive patterns for flexible and printed electronic devices is one of the most challenging steps in the whole process. Conductive patterns in electronic devices are used as electrodes, transducers, connecting links, and sometimes, also as the active sensing elements. Since the introduction of laser induced graphene (LIG), it has been explored to print electrodes and connecting patterns for various electronic devices and systems. This work focuses on an in-house developed laser printing system and the comparison of various electrical, chemical, and morphological properties of the resulting LIG patterns using CO2 and diode lasers. The system parameters including the laser power, relative printing speed, and the printing resolution were explored and optimized to achieve conductive patterns with varying properties suitable for various targeted applications. The fabricated patterns were characterized for their sheet resistance, surface morphology, chemical properties, and physical size and resolution. Continuous conductive patterns with sheet resistance in range of 11.5 Ω/□ to 43 Ω/□ were achieved using CO2 laser with a minimum achievable pattern width of ~ 180 μm while patterns with sheet resistance in range of 19 Ω/□ to 105 Ω/□ were achieved using diode laser with a minimum pattern width of ~ 190 μm. The chemical and morphological properties of CO2 laser-based patterns indicate the formation of 2D graphite sheets with high porosity and low O2 concentration while the diode laser-based patterns have a lower porosity and higher percentage of O2 indicating burning and formation of oxides. Various applications of both types have also been discussed based on their respective properties.


Introduction
Fabrication of electronic devices like sensors, transistors, LEDs, memristors, and other systems require multi-step and complex processes for various device parts like electrodes, transducers, connecting links, active sensing regions, etc. Printed and flexible electronics is becoming the future technology for device fabrication as it has revolutionized the prototyping and testing industry by providing a simple, cost effective, and quick solution for preliminary device testing and optimization as compared to the conventional lithography based device fabrication techniques. The major hurdle in the fabrication process of printed electronics is the fabrication of conductive patterns for electrodes, transducers, and connecting links [1][2][3][4][5]. A lot of research has been done to develop novel printing processes, and materials compatible with these processes, with an aim to reduce complexity, cost, and improve device performance [6][7][8]. Conventional conductive materials used in these processes for electrode fabrication are usually metallic or carbonbased including gold, silver, copper, graphene, graphite, activated carbon, etc. [9][10][11][12][13]. Printing of these materials is usually carried out in a solution form by first preparing an ink/suspension of these materials, and then developing patterns using computer based numerically controlled (CNC) systems. These materials are very expensive, difficult to synthesize, and usually require post-treatment at high temperatures (curing and sintering) to become conductive [14][15][16][17][18]. Carbon based materials have an advantage over the metallic materials in terms of lower cost, easier synthesis, and simpler post processing while they have disadvantages of lower conductivity, lower resolution, and lower robustness. Scientists are developing methods to improve the limitations faced in both categories of materials and Graphene/2D-Graphite has proven to be one of the most sought-after candidate in the Carbon-based materials. Graphene poses a high electron mobility of 200,000 cm 2 (V s ) −1 and a high specific surface area of 2600 m 2 g − 1 with a porous structure [19][20][21] making it ideal for applications like electrodes, energy-storing devices, sensors, etc. However, fabrication of Graphene using traditional methods either requires complex step-by-step chemical reactions, or very high temperatures [22][23][24].
In 2014, Lin et al., devised a new method of growing graphene flakes directly on Polyimide (PI) surface through laser scribing in the open air room environment. The researchers used an infrared (IR) carbon dioxide (CO 2 ) laser (10.6 μm wavelength) as the source that created very high surface temperature (pyritization) resulting in photothermal conversion of sp 3 -carbon atoms to sp 2 -carbon atoms [25]. Initially, it was assumed that LIG can only be grown on PI surface however, it was later discovered that many other carbonaceous polymers, nonpolymers, wood, metal/plastic composites, and biodegradable materials and foods may also be used as substrates for this purpose [26]. Moreover, after the initial studies on LIG produced using IR laser source, it was later observed that ultraviolet (UV) and visible lasers could also convert specific carbon precursors to LIG [26]. The carbon precursor substrates were transformed into LIG through photothermal effects for IR laser, photochemical effects for UV laser, and combination of photochemical and photothermal effects for visible laser. However, the dominating effect in all three types is the pyrolysis of the surface carbon compounds. The graphene formed as a result, referred to as laser induced graphene (LIG), exhibited excellent electrical conductivity with a highly porous structure similar to that of graphene foam produced via conventional techniques. Graphene foams were previously produced via conventional methods like chemical vapor deposition (CVD) and wet chemical processes but LIG provides a low-cost, facile, and scalable solution to the synthesis of high-quality non-toxic graphene. Another property of graphene induced via laser scribing is that it has many rings of five and seven carbon atoms resulting in a pentagon-heptagon hybrid lattice in contrast to the regular graphene where all the rings are six-membered. This happens due to the absence of rearrangement of rings through post-annealing that is a part of conventional graphene synthesis processes, and thus, such graphene is also referred to as "kinetic graphene" [27]. In addition, laser scribing using multi-axis computer numeric controlled (CNC) machines allows direct printing of graphene in any shape or pattern on any type of flexible or rigid surfaces. Since the discovery of LIG, and owing to the unique properties it possesses, it has been applied in a vast number of applications requiring high porosity and high conductivity at the same time [27,28] including the fabrication of electrodes [29,30], fuel cells [31], super-capacitors [32], batteries, humidity [33,34] and gas sensors [30,[35][36][37][38], mechanical sensors [39][40][41][42] and actuators [43,44], electrochemical [45,46] and biosensors [26,47,48], resistors, heaters [49], interconnects [50,51], etc. [52,53]. Furthermore, doped materials and composites have also been explored to produce doped LIG and LIG composites with certain enhanced properties for various applications like piezoresistive sensors [41], electrochemical sensors [31,46], ion detectors, and energy harvesters [54] etc.
Researchers have employed mainly two types of lasers to produce LIG including UV diode laser and IR CO 2 laser. Most of the works on exploring and tailoring the properties of LIG have been focused on studying the effect of the substrate materials on the morphological properties of the resulting LIG [26]. However, the LIG patterns produced via different laser types also slightly differ in their final properties due to the different mechanisms of conversion involved but no detailed investigation has been done in this regard. This work presents the comparison and optimization of electrical, chemical, and morphological properties of LIG patterns produced via IR CO 2 and UV diode lasers. The fabrication parameters like laser power and printing speed have been optimized for both cases and the results have been discussed in detail along with the discussion on the possible applications of the resulting patterns based on their respective properties.

Fabrication of LIG Patterns
To fabricate the LIG patterns, Kapton (PI) tape by Bedell with an overall thickness of 65 μm and a backing thickness of 25 μm, was used as the substrate mounted on Lasers in Manufacturing and Materials Processing (2023) 10:276-295 a glass slab. A commercial CO 2 CNC laser engraver (WR 4040 50 W) was used to produce CO 2 based LIG patterns while an in-house developed CNC system was used for the diode-based laser source. A continuous wave infrared (IR) CO 2 laser with a wavelength of 10.6 μm and a maximum possible power output of 50 W was used as the first source while a continuous wave UV diode laser with a wavelength of 450 nm, input power of 40 W, and a maximum possible optical power output of 5 W was used as the second source. The diode laser module was mounted on an inhouse developed computer numeric controlled (CNC) router as presented in Fig. 1. LaserGRBL software was used to communicate with and control the CNC router.
For the fabrication of CO 2 laser based LIG patterns, the output power of the laser was controlled by controlling the percentage current as per the hardware and software specifications of the system. According to the datasheet, the percentage current and output laser power have a linear relationship, thus allowing us to accurately set the power as per our requirements. Initially, the minimum power for graphenization was determined for CO 2 laser and it was found that no LIG was produced below 2.5 W. From 2.5 W to 3 W, LIG patterns were produced but the scribing speed had to be kept very slow that was not comparable to the speeds used at higher power levels. Two parameters were varied to study the effect of both, the laser power, and the scribing speed. The power of CO 2 laser was varied between 3 W, 3.25 W, and 3.5 W by adjusting the current at 6%, 6.5%, and 7% respectively while the stage speed was varied between 70 mm/s, 80 mm/s, 90 mm/s, and 100 mm/s. Increasing the speed more than 100 mm/s required the power to be increased further to produce conductive graphene while reducing the speed below 70 mm/s at the given power levels resulted in burning/evaporation of the substrate. Similarly, increasing the power beyond 3.5 W required the speed to be increased further to avoid burning/ evaporation of the substrate. The stand-off distance in all cases was kept at 20 mm as per the specifications of the manufacturer to get the best focus. Multiple squarish patterns with dimensions of 12 mm by 10 mm were produced for various speeds and power combinations for both types of lasers. The minimum width achieved for LIG conductive pattern using CO 2 laser source was 180 μm showing an excellent printing resolution of the system. A summary of fabrication parameters for both laser systems is presented in Table 1.
In case of the diode laser, the minimum power for graphenization was found out to be 1.9 W below which no LIG was produced at any speed. To study the effects of laser power and speed, the laser power was varied between 2 W, 2.5 W, and 3 W by changing the duty-cycle of the pulse width modulated (PWM) control waveform between 40%, 50%, and 60% respectively, while the relative printing speed was varied between 10 mm/s, 20 mm/s, and 30 mm/s at all the set laser power values. The stand-off distance was adjusted to achieve the best focus for the round laser spot through visual observation using a UV laser filter. The X-Y stage travel resolution was 265 steps/mm while the maximum rate achievable was 2400 mm/min or 40 mm/s for the in-house developed stage. The stage speeds had to be kept different than those selected for CO 2 laser system because higher stage speeds in case of diode laser resulted in no graphenization and no LIG was produced because the effective incident power is dependent on both the beam power and the exposure time that is determined by the stage speed. Shorter exposure for a shorter wavelength laser resulted in not enough power for pyritization. This had to be addressed by reducing the printing speed to allow enough exposure time for graphenization of the surface. This also shows that much faster printing speeds can be achieved using the CO 2 laser as compared to the diode laser. A minimum line width of 190 μm was achieved for the conductive LIG patterns using the diode laser-based system as shown in Fig. 2. This shows that the printing resolution of our in-house developed diode based system is comparable to that of the commercial CO 2 based system and a minor difference is due to some vibrations as well as the possible human errors in manual beam focusing and different beam dimensions for the two laser sources. Laser ablation of polyimide occurs due to both photothermal and photochemical processes. In case of CO 2 based laser source with a longer wavelength, photothermal effect is dominant, while in case of diode-based laser with shorter wavelength, both photothermal and photochemical processes result in the formation of graphene with a relatively lower contribution of photothermal effects [25,55].

Characterizations
The electrical, chemical, structural, and morphological properties of the LIG patterns were studied for comparison and optimization. Sheet resistance of the conducting patterns was calculated by connecting two equal sized flat electrodes across the LIG surface with the distance between the electrodes equal to the electrode length resulting in a unit square of 10 mm x 10 mm. Pressure was applied perpendicular to the plane to ensure good contact of electrodes and the LIG patterns. The physical morphology of fabricated LIG patterns was investigated using field emission scanning electronic microscopy (FESEM) performed through Carl Zeiss EVO 18 with integrated energy dispersive spectroscopy (EDS). EDS was also studied to investigate the elemental composition of the LIG patterns. Further chemical properties and energy states of the LIG patterns were studied using LabRAM HR Raman spectrometer to determine the type of chemical structure of the patterns. Based on their properties, the patterns were also employed in various applications including flex sensor, inter-connect conducting pattern, and capacitor electrodes. The flex sensor was characterized by manually flexing and relaxing the pattern while recording the resistance in real-time using a USB connected AT-825 LCR meter. The capacitance Fig. 2 Specifications of the two systems based on CO 2 and diode laser sources used for the fabrication of conductive LIG patterns was also measured using the same LCR meter at various frequencies. The conductive interconnects were tested by printing a simple LED circuit with a push button.

Morphological and Chemical Properties
The surface SEM results showing the physical morphology of LIG patterns produced at different powers and printing speeds via CO 2 laser are presented in Fig. 3. The porosity (surface area to volume ratio) of the samples was qualitatively approximated through contrast analysis of the SEM images for a uniform area of observation (~ 100 μm x 75 μm) for all samples. Two factors were considered to approximate the porosity including the average pore size and the number of pores in the observable area. The results presented in supplementary Figs. S1 and S2 show that the number of pores increases with increasing laser power while the average pore size (total area/number of pores) decreases confirming that a greater number of 2D graphite flakes are produced at higher power. This implies that the surface of the LIG patterns becomes increasingly porous (larger surface area to volume ratio) with increasing laser power, depicting direct relationship between power and porosity. On contrary, the porosity decreases with increasing printing speed because the effective excitation laser power and the exposure time, that is inverse of the printing speed, are directly related. The surface morphology results indicate that increasing the exposure time or effective power results in the formation of better 3D structure of the films based on 2D graphite with single layer graphene flakes randomly stacked across the thin film. Better porosity indicates larger surface area to volume ratio making the films ideal for use in batteries, super capacitors, and as bio and gas sensing active films [26,31,32,[45][46][47][48].
Similar behavior can be observed in the SEM results of LIG patterns produced via UV diode laser presented in Fig. 4. The porosity of the films increases with increasing laser power and decreasing printing speed (longer exposure time). The results, however, differ from the CO 2 based patterns in terms of overall lower porosity of the LIG films for the diode laser based patterns. Figure 4a shows that the pattern with the lowest power and maximum printing speed (minimum effective power) has almost a non-porous morphology. The carbon produced as a result of laser ablation at lower power using diode-based laser is in the form of large sheets similar to amorphous carbon usually resulting from pyrolysis [55]. The surface temperature is not high enough to convert sp 3 -carbon atoms to sp 2 -carbon atoms as in case of 2D graphite. The lower temperature (800-1500°C) results in amorphous carbonization of PI, while the photothermal laser ablation at higher power and high localized temperatures (> 2500°C) would break the C-O, C = O, and N-C bonds, and the aromatic compounds then rearrange to form graphitic structures [25,55]. It is interesting to note here that the CO 2 laser did not result in any carbonization at power lower than 2.5 W while above 2.5 W, the patterns produced were all porous with morphology On the other hand, the diode laser resulted in carbonization of the PI substrate even at 1.9 W but the resulting films were not porous indicating amorphous carbon similar to the results of Fig. 4a. As the effective power was increased, however, the films became more and more porous indicating the formation a greater number of 2D graphite flakes resulting from a higher localized surface temperature and longer exposure [25]. It was observed that the lower porosity patterns could be more suitable for applications like interconnects printing, mechanical sensors, resistors, etc. [39][40][41][42][49][50][51] To further investigate the structural properties and composition of the fabricated patterns, EDS was performed for both CO 2 and diode laser based LIG samples prepared at different laser powers. Results of EDS for CO 2 laser-based samples presented in Fig. 5 show that for both maximum-power-minimum-printing speed and minimum-power-maximum-printing speed, the samples were fully carbonized to dense 2D graphite flakes with no oxidation and damage to the samples. It further confirms the SEM results of Fig. 3 where all samples prepared using CO 2 laser showed similar porous morphology depicting the formation of graphene.
EDS results for the diode laser-based samples are presented in Fig. 6. The results show that for minimum power (Fig. 6a), the samples had a very high degree of oxidation, and also, Si from the bottom glass slab was detected at a high percentage that is probably due to the thin layer of ash like carbon formed due to pyrolysis instead of a dense sheet of 2D flakes [55]. The level of oxidation and the percentage of Si keep on decreasing for samples fabricated with increasing laser power, with the results coming closer to those produced via CO 2 laser having a dense sheet of 2D graphite flakes. This confirms the observations made based on the visual SEM results presented in Fig. 4.
To corroborate the structural properties of the prepared samples, RAMAN spectroscopy was performed for the samples fabricated at maximum and minimum powers for both types of laser sources with the results presented in Fig. 7. The RAMAN spectrum (black) of LIG pattern produced via CO 2 laser at maximum power of 3.5 W and minimum speed of 70 mm/s shows three sharp peaks at 1384 cm − 1 , 1580 cm − 1 , and 2696 cm − 1 with intensities of 3194, 3930, and 2400 respectively.  Similarly, the RAMAN spectrum (Blue) of the CO 2 laser-based sample fabricated at minimum power (3 W, 100 mm/s) shows the same peaks at 1350 cm − 1 , 1580 cm − 1 , and 2694 cm − 1 with respective intensities of 2719, 3267, and 1651, a bit lower when compared to those for the maximum power based samples, but pretty close. These peaks correspond to the signature D-band, G-band, and 2D-band of graphene respectively [56]. The D-band indicates defect sites associated with higher energy sp 3 while G-band shows the lower energy sp 2 carbon bonds [56]. The intensity ratio of D and G peaks (I D /I G ) can be used to determine the degree of graphenization that is inversely proportional to the in-plane crystallite size as per Tuinstra-Koenig relationship [29]. The ratio was calculated to be 0.81 in case of maximum power CO 2 laser based samples while it was 0.83 for minimum power CO 2 laser based samples showing a high degree of graphenization in all cases of CO 2 laser. Furthermore, a very strong 2D band, that is the second harmonic of D band, was observed centered at 2696 cm − 1 which is similar to that for a single layer graphene but relatively a bit wider indicating that the 2D flakes are randomly stacked along the c-axis [25]. The RAMAN spectra for the samples fabricated using the maximum power (3 W, 10 mm/s) diode laser (red) and minimum power (2 W, 30 mm/s) diode laser (green) look more similar to the spectrum of glassy/amorphous carbon than that of graphene [57]. The overall intensities of all the three bands are significantly lower than those in the CO 2 laser based sample. Two particular observations that can be made in the spectrum of minimum power sample (green) include the slightly higher intensity of the D band as compared to the G band and the relative widening of the much weaker 2D band. The I D /I G ratio for the minimum power (green) diode laser based sample comes out to be 1.03 which clearly shows that the graphenization was incomplete, as also indicated by the much smaller 2D band. However, the I D /I G ratio for the maximum power (red) diode laser based sample comes out to be 0.86 that is similar to that of CO 2 laser based samples. The reason for better graphenization in case of IR laser source as compared to the UV laser source is because in case of IR laser, with a much longer wavelength, LIG is produced via photothermal effects in contrast to photochemical effects dominant in case of a UV laser with a much smaller wavelength. Pyritization is completed in case of photothermal conversion while remains incomplete in case of photochemical conversion for smaller wavelengths and shorter exposure time. This also leads to the formation of oxides and incomplete breaking of Carbon -Oxygen bonds, resulting in the overall lower intensity and wider Carbon peaks for UV laser based samples. Higher power and longer exposure times for the UV diode laser based samples also result in better graphenization and somewhat complete pyritization with results coming closer to those produced by CO 2 laser.

Electrical Properties and Applications
Electrical properties of the fabricated samples were investigated by measuring their sheet resistance. The results presented in Fig. 8 show the relationship of sheet resistance with the laser power and the printing speed for both the diode and CO 2 lasersbased samples. Figure 8a shows that the sheet resistance of CO 2 based samples increases linearly with increasing printing speed that is expected as the exposure Lasers in Manufacturing and Materials Processing (2023) 10:276-295 time is reduced and the effective power in a unit time decreases. The results of Fig. 8b show that the sheet resistance decreases non-linearly with increasing power for CO 2 laser. The results are in concordance with the results of morphological characteristics determined by the SEM and chemical characteristics determined by the RAMAN spectroscopy. The morphological characterizations show that the number of flakes increases with decreasing printing speed and increasing laser power, thus predicting lower sheet resistance for the samples fabricated at higher effective power due to a greater number of parallel current paths available because of a greater number of flakes. Similarly, the EDS results also predict the same electrical characteristics as the carbon content is higher in samples fabricated at higher laser power with lower oxidation rates predicting lower resistance for samples fabricated at higher effective power. The results of RAMAN spectroscopy also indicate that better pyritization is achieved at higher power and lower speeds resulting in better graphenization and more carbon content, thus depicting lower resistance for the samples fabricated at higher laser power. Similarly, the results in Fig. 8c and d show the same trend of sheet resistance versus printing speed and the laser power for the diode laser-based samples. The resistance in case of diode laser-based samples show linear trends for both printing speed and laser power. Minimum sheet resistance of 11.5 Ω/□ was observed for the CO 2 laser-based samples at a maximum laser power of 3.5 W and a minimum printing speed of 70 mm/s while maximum resistance of 43 Ω/□ was observed for minimum power of 3 W and maximum speed of 100 mm/s. Similarly, minimum sheet resistance of 19 Ω/□ was observed for the diode laser-based samples at a maximum laser power of 3 W and a minimum printing speed of 10 mm/s while maximum resistance of 105 Ω/□ was observed for minimum power of 2 W and maximum speed of 30 mm/s. Reducing the speed further or increasing the laser power resulted in burning of the samples while the resistance was not reduced any further. Also, for the CO 2 laser, reducing the power below 2.4 W resulted in failure to achieve any carbonization. Increasing the speed, however, resulted in further increase of resistance but the limit was not of interest as higher resistance patterns, with much flexible tuning option for higher resistance values, could be obtained using diode laser. The minimum laser power for diode laser to achieve enough carbonization resulting in conductive patterns was found to be 1.9 W.
The sheet resistance values and the morphological and chemical properties obtained for both types of samples open up the possibilities for these samples to be used in various applications. As mentioned in the discussion on the SEM results, the LIG patterns obtained via CO 2 laser ablation have a higher porosity owing to the greater number of 2D graphite flakes and they are more suited for energy storage applications like electrodes for batteries, supercapacitors, and working electrodes for electrochemical and biosensors. Same conclusions can be deduced from the results of the sheet resistance as the electrodes must have a low sheet resistance for a better performance. Similarly, for the diode laser based LIG samples, the porosity is low while the resistance is higher. The conductive patterns are suited for applications like flex sensors (higher resistance for better sensitivity), printed resistors (tunable resistance values), heaters (tunable heating power), and inert inter-connects (fabricated at higher laser power for better conductivity). Some of the possible applications of patterns fabricated through both types of lasers were implemented, and the results achieved for the same devices fabricated using the two different laser sources were compared for better understanding and affirmation of the hypothesis. Table 2 shows the various devices and their results, fabricated using the two types of lasers, to compare their performance and confirm their efficacy for the above suggested possible applications. The fabricated devices included an interdigitated electrode (IDE) surface capacitor, a u-shaped flex sensor, and a simple interconnect conductive electrode pattern.
Different categories of devices were selected and their properties and performance were compared in this work in view of the different requirements posed by each device category. For the conductive interconnects, lower resistance means better performance. Similarly, for capacitors and battery storage devices, higher capacity means better performance. In case of sensors, higher sensitivity is preferred. The aim was to compare the performance of these device types fabricated through the two different laser sources. The structure and parameters of the selected devices 1 3 Lasers in Manufacturing and Materials Processing (2023) 10:276-295 however were not investigated in depth and were not optimized from individual device perspective as the aim of this work was not device fabrication but rather investigation of difference in performance of same devices fabricated through different laser types. The results presented in Table 2 ascertain that the performance of conductive interconnect electrode patterns for the CO 2 laser based samples is better as they pose much lower resistance value when compared to that of the diode laser based electrodes, both being fabricated at the maximum respective laser powers for minimum resistance. Similarly, the capacitance of the CO 2 laser based IDE capacitor was three times as that of the diode laser based capacitor for the same dimensions and electrolytes owing to the higher conductivity and larger surface area of the electrodes for CO 2 laser based device. The results of flex sensor fabricated through diode laser show that it had much higher intrinsic resistance when compared to its CO 2 laser based counterpart fabricated using similar printing parameters and thus had much higher sensitivity as expected. These device results may look inferior to the similar devices already reported in the literature but that does not affect the outcome of this work as the focus here is not on individual devices. It can thus be concluded that for the applications requiring higher and more tunable resistance value of the conductive patters, diode laser is a better choice, while for the applications requiring low resistance and large surface area with more porosity, CO 2 laser is recommended.

Conclusions
Conductive laser induced graphene (LIG) patterns were fabricated using two different types of lasers including a UV diode laser and an IR CO 2 laser. The fabrication parameters including laser power, relative printing speed, and printing resolution were optimized to achieved conductive patterns with varying morphological, chemical, and electrical properties. LIG patterns fabricated using diode laser showed lower film porosity with lower surface area as compared to those fabricated using CO 2 laser due to lower number of graphene flakes available. Resultingly, the patterns fabricated using diode laser had a higher sheet resistance (19 Ω/□ to 105 Ω/□) as compared to their counterparts fabricated using CO 2 laser (11.5 Ω/□ to 43 Ω/□) due to lower number of paths available for the flow of current because of lower number of flakes. The reason for a lower resistance and higher porosity of CO 2 laser based patterns is the formation of a large number of highly conductive 2D graphite flakes due to pyritization resulting from photothermal effects in case of a longer wavelength IR laser. For the diode laser, surface carbonization at smaller wavelength resulted in incomplete pyritization and oxidation of the patterns with the graphite produced not in the form of distinct 2D flakes. A greater number of 2D flakes was observed in case of higher power for diode laser resulting in morphological and electrical properties approaching closer to those of CO 2 based laser but burning was observed if power was further increased for diode laser. It was also observed that the resistance of diode laser based LIG patterns was much more tunable and much higher values of resistance could be achieved for the patterns with same dimensions when compared to the CO 2 laser based patterns. It was thus theorized that the diode laser based patterns were more suitable for applications like fabrication of resistors, resistive heating elements, and resistive sensors including flex sensors, temperature sensors, etc. where higher intrinsic resistance is preferred for a better sensitivity. Similarly, the CO 2 laser based patterns were recommended for applications where low resistance is required like electrodes and interconnects fabrication, and for areas where high surface area is desirable like electrochemical sensing, batteries, and supercapacitors. Sample devices were fabricated using both laser types for some selective applications including interdigitated capacitor, flex sensor, and conductive interconnect; and the results confirmed better performance of CO 2 laser based patterns for capacitor and interconnects while the sensitivity of flex sensor was much better for the diode laser based devices as expected. Data Availability There are no datasets involved. Any other data will be made available on request.

Declarations
Ethical Approval There were no animal or human studies involved in this work.
Competing Interests It is declared that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.